HR: 1330h
AN: V22D-0619    [PDF]
TI: Stability relations of Ti-bearing assemblages in UHP marbles from the Kokchetav Massif
AU: * Kikuchi, M
EM: kikuchi@earth.edu.waseda.ac.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Ogasawara, Y
EM: yoshi777@waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AB: Stability relations of Ti-bearing phases (rutile, titanite, Ti-clinohumite) in UHP marbles from the Kokchetav Massif, northern Kazakhstan were well analyzed by the phase relations in the model system CaO-MgO-SiO$_{2}$-TiO$_{2}$-CO$_{2}$-H$_{2}$O. Three types of UHP marbles have been described in the Kumdy-kol area: diamond-bearing dolomite marble (type-A), diamond-free Ti-clinohumite-bearing dolomitic marble (type-B), and diamond-free calcite marble (type-C) (Ogasawara et al., 2000; 2002), and they contain a distinct Ti-bearing phase, rutile, Ti-clinohumite, and titanite, respectively. The characteristics of these three marbles are as in the followings. Type-A: Dominant carbonate is dolomite. Abundant microdiamonds are included in garnet and diopside. Diopside-dolomite tie-line was stable. Diopside contains K-bearing silicate lamellar. The peak assemblage was dolomite + diopside + aragonite + garnet + rutile + diamond. Type-B: High MgCO$_{3}$-calcite (originally aragonite + dolomite) is distinct. No diamond occurs. Diopside lacks lamellar texture. The peak assemblage was aragonite + dolomite + garnet + Ti-clinohumite or forsterite. Type-C: Only carbonate phase is calcite (originally aragonite). No diamond occurs. Titanite contains coesite exsolution lamellae and indicated supersilicic compositions stable at UHP conditions (P $>$ 6GPa). Diopside contains phengite and K-feldspar lamellae. The peak assemblage was aragonite + diopside + K-feldspar + garnet + titanite. The solid-solid reaction, CaTiSiO$_{5}$ (titanite) + CaMg(CO$_{3}$)$_{2}$ (dolomite) = CaCO$_{3}$ (aragonite) + CaMgSi$_{2}$O$_{6}$ (diopside) + TiO$_{2}$ (rutile), controlled the stability of Ti-bearing phases in calcite marble and dolomite marble. At UHP conditions, aragonite + diopside + rutile assemblage is stable compared with titanite + dolomite, and divides the compositional space into dolomite-free and dolomite-bearing tetrahedrons in the model system CaO-MgO-SiO$_{2}$-TiO$_{2}$. The presence of titanite in calcite marble indicates that the P-T condition was located in the right-hand side of the decarbonation reaction aragonite + rutile + coesite = titanite + CO$_{2}$. The lack of rutile in calcite marble can be explained by low bulk TiO$_{2}$ contents of this marble. The mineral assemblages in dolomitic marble require extremely low-X$_{ CO2}$ conditions compared with those in dolomite marble because the aragonite + Ti-clinohumite tie-line was stable (Ogasawara et al., 2000). The continuous sample from the Kumdy-kol (sample no. Y665: dolomite marble and dolomitic marble) suggests X$_{ CO2}$ heterogeneity in cm scale under UHP conditions. The stability of Ti-clinohumite was controlled by the reaction: dolomite + diopside + TiO$_{2}$ + H$_{2}$O = Ti-clinohumite + aragonite + CO$_{2}$. All these results related with Ti-bearing phases were consistent with the previous works on the Kokchetav UHP carbonates (Ogasawara et al., 2000; 2002). References: Ogasawara, Ohta, Fukasawa, Katayama and Maruyama (2000) {\it The Island Arc}, {\bf 9}, 400-416. Ogasawara, Fukasawa and Maruyama (2002): {\it Am. Mineral}. {\bf 87}, 454-461.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting